The RadNet (formerly Environmental Radiation
Ambient Monitoring System) database is a rich source of radiation
data from the environment around us. Used properly, the data
can tell us a great deal. One example of its use is the long-term
trends radionuclides in certain media. Below is the trend
of tritium in precipitation.

Trend of H-3 (Tritium) in Precipitation

This trend is from monthly concentrations compiled between 1978-2001.
(You can see the data on the Envirofacts website.) Data came from samples of rainfall,snow, or sleet that fell throughout the month and were then combined into a single sample for each station.

Each sample was analyzed for tritium, and other radionuclides. The graph below shows the national average tritium concentration in precipitation. You can see from the graph that tritium concentration between 1978 and 1980 was in the range of 350 pico Curies per liter (pCi/l) and spiked to about 680 pCi/l during the last above ground nuclear test to levels below what instruments can detect.